well I've played more games than anyone else who has posted on this thread, so clearly I'm the best one here, right?
glad we agree.
now, soloing comes down to a few basic tricks. If you are a SC leader, and well into a game, bring one of the smaller powers under your wing, support him, make him feel like you want to move for a two way draw.
Then, set up DMZs that you say 'are for both of our sakes' but really only ensure your safety. If he has a unit placed where it can block any stab you have easily, tell him that it's *making you unsettled, and if he could move it back or to another territory* (hint towards a place that looks fine, but is actually much worse.
After applying seasonings to the delicious turkey of betrayal, eat that bird. Many people on this site have a natural carebear reflex they resort to, so you can use that.
if you are NOT the SC leader, you need to be all bolshevik, and rally people against the main power. If the main power is close to you, the enjoy the sweet deluge of SCs coming your way, if the main power is away from you, then stab someone close to you who is MOVING towards the world power. (easiest in Anc Med or Classic, much harder in world where they're all spread out). People on this site are also a bunch of die hards who always have stars in their eyes, so spew out the 'band of brothers' and emotionally charged crap. Be prevalent in chat, and clinical in betrayal.
The last piece of advice I can give you: play gunboat occasionally. Although it lacks the 'diplomacy' part of the game, it gives you much experience introspectively on people's moves and strategies when moving forward. It helps you develop great ability in your own movements, and can pick people out in full press games, revealing ulterior intentions. You need to know the best ways to move your units, the time to convoy, the time to support yourself into your own territory as that unit is able to bounce support, the time to stab just to gain territory and positioning.